The invention relates to a filter for a fluid-conducting pipe, in particular for a gas-conducting pipe of an internal combustion engine.
A filter of the present type comprises a self-supporting areal filter element which may be of inherently stable, single-layer or multi-layer construction, or else may be comprised of a supporting structure which is permeable over its full area and which has seated thereon a filter layer which is not imperatively inherently stable. This filter element is, along its circumference, fixed directly or indirectly in the fluid-conducting pipe. Here, said filter element covers a free cross section of the pipe, wherein its circumference circumscribes a central filter plane.
The preferred field of use of a filter of the present type is a gas-conducting pipe of an internal combustion engine, specifically a line for exhaust-gas recirculation for the reduction of the emissions of nitrogen oxides that are generated during the combustion of fuel in Otto-cycle engines, diesel engines, gas turbines, heating boilers, etc. The thermal loads in the fluid-conducting pipe in which the filter is to be used are correspondingly high.
In the case of external exhaust-gas recirculation, which is obligatory in particular in motor vehicle internal combustion engines owing to the stringent exhaust gas regulations, the exhaust tract and the intake tract of the engine are connected by a line. This line may run in the cylinder head or may be a separate line outside the engine block. If the air inducted by the internal combustion engine is, in this way, admixed to the exhaust gas, the oxygen concentration of the mixture supplied to the engine decreases, which lowers the combustion temperature in the cylinders of the engine. Since the formation of nitrogen oxides is highly temperature-dependent, the nitrogen oxide formation can be reduced in this way. To increase the efficiency of this process, the recirculated exhaust gas must however be supplied at as cool a temperature as possible to the intake tract, which may be realized through active cooling of the recirculated exhaust-gas flow or by means of an extraction of the exhaust gas to be recirculated at a point of the exhaust system situated further downstream, at which the exhaust-gas temperatures have already decreased to a considerable extent.
In particular in the case of modern diesel engines, it is conventional to use hybrid exhaust-gas recirculation systems in which a part of the exhaust gas is extracted for recirculation close to the engine (high-pressure exhaust-gas recirculation), and a part is extracted downstream of the catalytic converter or diesel particle filter (low-pressure exhaust-gas recirculation).
It is a problem here that, via the low-pressure exhaust-gas recirculation, ceramic particles of the catalytic converter or diesel particle filter can pass into the intake tract of the engine. Furthermore, via the high-pressure exhaust-gas recirculation line, soot particles can be transported into the intake tract. Ceramic particles can cause damage to the moving components of the engine, in particular to the compressor wheel of the turbocharger that is normally provided. Also, however, soot particles in the recirculated exhaust-gas flow can damage a turbocharger or other components such as a cooler for the recirculated exhaust-gas flow, because the soot particles act as condensation nuclei and initiate the formation of condensate. The condensate which forms then leads to so-called varnishing of the components.
In exhaust-gas recirculation systems of internal combustion engines, it is therefore essential to filter the recirculated exhaust-gas flow. This is realized in the prior art by a filter element which is comprised substantially of a metal nonwoven. In particular due to the thermal loading of the filter, inserting the filter into the fluid-conducting pipe such that it is permanently and functionally fixed is not without problems.
Previous solutions, for example according to DE-A-10 2010 050 393, use for this purpose a pipe connector with a cover which is provided with a particular hole pattern for the passage of the fluid to be filtered. The filter element is positioned on said cover and fixed by a second cover which has an identical hole pattern to the first cover and which is oriented in alignment therewith. The filter element is enclosed between the first and the second cover, such that the fluid which flows through the holes in the two covers must pass through the filter element and be filtered there. The first and the second cover are fixed to one another by a welded connection.
The manufacture of a filter of this type according to the prior art, and the installation in particular in an exhaust-gas recirculation line of an internal combustion engine, are disadvantageously cumbersome due to the necessary exact orientation of the components with respect to one another and in particular owing to the necessary welding. Furthermore, the weldability of the components demands a minimum wall thickness thereof, which keeps the material costs high. Finally, in the preferred field of use in a gas-conducting pipe of an internal combustion engine, problems arise as a result of the high thermal loading of the filter, because the hole patterns of the two covers that are used are self-evidently provided with very narrow webs between the holes so as not to hinder the fluid flow to any great extent, which webs are accordingly susceptible to hot cracking.
It is pointed out at this juncture that the present invention is not restricted to filters for use in gas-conducting pipes of internal combustion engines, even though this is the preferred use and the above-described prior art was the starting point for the considerations relating to the invention. Other uses of filters for insertion into fluid-conducting pipes present similar problems.